A manipulator for a system for producing bent sheet-metal articles

Abstract: A manipulator for gripping and moving an initially flat piece of sheet metal to be subjected to bending operations by means of a bending machine, comprising a suspension head (76) which can allow the piece (180) to perform small movements in space with every degree of freedom except for rotation about an axis perpendicular to the plane of the undeformed piece. The suspension head (76) has rapid engagement and release means for gripping and releasing a gripper (98) adapted to grip a piece of sheet metal (180). The suspension head (76) comprises at least one pair of engagement fingers (260) which are movable axially in a conical seat (258) housing a shank (106) of the gripper (98), the engagement fingers (260) being carried by a movable piston (252) operated by pressurised fluid, and cooperating with a fixed shaft (262) which moves the engagement fingers (260) apart resiliently.

Description: The present invention relates to the production of bent sheet-metal articles and concerns a manipulator for an automatic bending system, of the kind defined in the preamble of Claim 1. Background Art The invention has been developed to solve the problem of the bending of sheet-metal articles of complex shapes which are frequently used in machines such as photocopiers, facsimile machines and various electronic devices. These products are subject to rapid development and manufacturer therefore often changes models from one year to another. Each new model is the product of a redesign, even as regards the various sheet-metal articles which it contains. These bent sheet-metal articles are therefore produced on a relatively small scale and thus do not justify complex and expensive tools and dies. A system for producing bent sheet-metal articles known from U.S. Patent No. 4,991,422 departs radically from previously existing bending systems which use bending presses with fixed frameworks and linear, V-sectioned punches and dies which are movable vertically towards and away from each other. The system described in the U.S. Patent mentioned above provides for a piece which is to be bent to be supported by a manipulator so that a region of the piece which is to be bent lies in a vertical suspension plane. The bends are effected by means of an oscillating bending machine having two tools which can be disposed in any configuration relative to the piece to be bent. The piece is supported by the manipulator in a manner such that it can perform movements of limited extent with five degrees of freedom, excluding rotation about an axis perpendicular to the plane of the undeformed piece of sheet metal. The bending machine also has a device for the rapid replacement of the tools, using two rotary turrets carried at the ends of a C-shaped tool-holder structure. Disclosure of Invention The object of the present invention is further to develop the bending system, the essential elements of which are described in the aforementioned U.S. Patent No. 4,991,422, and in particular to improve the manipulator which has a critical role in the practical application of this innovative bending system. The object of the invention is achieved by means of a manipulator having the characteristics defined in claim 1. The present invention will now be described in detail with reference to the appended drawings, provided purely by way of non-limiting example, in which: Brief Description of Drawing Figure 1 is a schematic, perspective view showing a system according to the invention as a whole, Figures 2 to 4 show in section, in elevation, and in plan, a gripper for fitting on to a piece of sheet metal, Figures 5 is a schematic view showing the operating principal of the suspension head, of a manipulator according to the invention, Figure 6 is a perspective view of the suspension head, Figures 7 and 8 are schematic, longitudinal sections of the head of Figure 6 taken in two perpendicular section planes, Figure 9 is a section similar to that of Figure 8 with the addition of clamping devices, Figures 10 and 11 are sections, on enlarged scale, of the part indicated by the arrow X in Figure 9, showing the device for the engagement of a gripper, Figure 12 is a section showing the clamping device indicated by the arrow XII in Figure 11, on an enlarged scale, With reference to Figure 1, a system for producing bent sheet-metal articles from blanked or laser-cut pieces of sheet metal of shapes corresponding to the development in a plane of the articles to be produced, is generally indicated 50. The system 50 comprises a station 52 for positioning the pieces of sheet metal, a measurement and storage station 54, a bending station 56, an output station 58 and a device 60 for the automatic replacement of the tools, with a respective tool store 62. The bending station 56 comprises a cartesian manipulator 64 including a vertically-movable device 66 carried by a carriage 68 movable along a beam 70 which in turn is movable along guides 72 of a portal structure 74. The movable device 66 of the manipulator 64 carries a suspension head 76 which will be described in detail below, for holding vertically, by means of a gripper, a piece of sheet-metal to be bent. An important characteristic of the system according to the present invention is constituted by the fact that the gripper is not connected to the suspension head of the manipulator 64, but is connected to the piece to be bent. The suspension head 76 of the manipulator 64 has the characteristic that it supports the piece in a manner such that it floats freely, so that the piece is free to perform movements of a limited extent during bending. In order to execute each bend, the manipulator 64, which is controlled by a conventional control unit 78, positions the piece in a position which is determined on the basis of a program established in dependence on the geometrical shape of the piece to be worked. The bending station 56 also comprises a bending machine 80 comprising a tool-holder structure 82 having a punch 84 and a die 86 which cooperate with each other. As will be described in detail below, the tool-holder structure 82 is rotatable about an axis which passes through the bending line defined by the vertex of the V-shaped punch 84 and can also pivot about a horizontal axis perpendicular to the aforesaid axis of rotation. It will therefore be appreciated that the punch 84 and the die 86 can be disposed in any position relative to the piece to be bent. The rotary and pivoting movements of the tool-holder structure 82 are brought about by the control unit 78 on the basis of a predetermined program. It is important to underline that the precision of the positioning of the piece in space, like the precision of the positioning of the tools, is of decisive importance since, unlike conventional bending methods, there are neither mechanical abutments to define the position of the piece nor systems for measuring the position of the piece relative to the bending machine. The necessary precision in the positioning of the piece is achieved by virtue of a preliminary determination of the relative piece-manipulator position and of the precise control of the relative manipulator-bending machine positions, which is achieved by virtue of the operating precision of the manipulator 64 and of the bending machine 80. The operating principle upon which the bending system according to the invention is based thus consists of the positioning of a piece of sheet metal in a predetermined region in space with great precision and repeatability, and of the modification of the positions of the bending tools relative to the piece, with a corresponding degree of precision and repeatability, by a movement of the bending machine, so as to execute the bend in the desired region. For further clarification as regards the operating principle of the system according to the present invention, reference should be made to U.S. Patent No. 4,991,422. The main characteristic of the system according to the present invention is its ability to work on extremely small batches (even a single piece) of pieces with different geometrical shapes, solely by means of the selection of a different working program, without carrying out tooling operations. A first problem which had to be solved in order to achieve a high degree of flexibility of the system was that of devising a unit for loading the pieces which enabled shaped pieces of sheet metal of complex shapes and extremely variable dimensions to be stored and subsequently gripped by the suspension head 76 of the manipulator 64. With conventional grippers carried by the head of the manipulator, in addition to the difficulty of producing a universal gripper which can grip pieces of different geometrical shapes, there is the problem of the precise positioning of the piece relative to the gripper and hence relative to the locating system of the manipulator, that is, the problem of how to position the piece precisely on a loading device which, at the same time, can be adapted to pieces which differ greatly in shape and size. In the system according to the present invention, these problems have been overcome by virtue of the fact each piece is associated with its own gripper which is fitted on to a predetermined region of the undeformed piece of sheet metal. The gripper is fitted on to the piece of sheet metal in the positioning station 52 in the manner which will be described in detail below. The pieces, with their respective grippers, are disposed in a store 90 to await transfer to the bending station 56. The store 90 can easily house pieces of different geometrical shapes without the need for any tooling, by virtue of the use of the gripper. In fact, the store 90 can accommodate a certain number of grippers which are inserted in a corresponding number of forks forming part of the store. The pieces of sheet metal, each gripped by its own gripper, are disposed vertically, suspended by the gripper itself, and thus have no direct connection with the store 90 which is completely independent of the shapes of the sheets. After each piece of sheet metal has been provided with its gripper, the piece is subjected to a measurement step carried out by means of a conventional feeler 88. The data detected by this measurement are processed and stored by the control unit 78 which establishes the link which exists between a locating system fixed relative to the piece and a locating system which is fixed relative to the gripper and, consequently, is fixed relative to the suspension head 76 of the manipulator 64. Small corrections can thus be made to the program controlling the manipulator 64 to compensate for errors in the positioning of the piece relative to the gripper. The piece may be measured when it is already connected to the suspension head 76 of the manipulator 64. In this case, the feeler 88 must be movable in order to enter and leave the working area. If the cycle for the working of the piece provides for the gripping region to be changed after some bends have been effected, a new measurement can be made after the piece has been gripped in the new position. Alternatively, and as shown in Figure 1, the measurement may be effected outside the working area, without affecting the time taken by the bending cycle, whilst the piece is supported by an auxiliary manipulator 89. An alternative could be that of ensuring sufficient precision in the positioning of the piece relative to the gripper at the moment when the gripper is fitted on to the piece so as to avoid the measurement step altogether. Upon completion of the bending operations, the manipulator 64 brings the worked article to an output station 58, shown schematically by means of a belt conveyor. There may be a device 92 in the output station 58 for removing the grippers from the sheet-metal articles. The manipulator for gripping and moving a piece of sheet metal As has been seen above, each piece to be bent is fitted with its own gripper. Referring to figures 2, 3 and 4, the gripper 98 is a purely passive element, that is, it does not have opening and closure mechanisms of its own. The gripper 98 is constituted by a monolithic metal body having two resilient arms 100, to the ends of which two plates of frictional material 102, between which a piece of sheet metal can be gripped, are fixed. A cavity 104 (Figure 2) is defined between the two arms 100 for housing a mechanism for moving the arms 100 apart resiliently. The gripper 98 has a shank 106 with a tapered outer surface for engagement on the suspension head of the manipulator. The shank 106 has a through-hole 108 which communicates with the cavity 104 and has a seat 110 for engagement by means which connect the gripper 98 to the manipulator head. The gripper 98 also has a pin 112 for the angular location of the gripper relative to the manipulator head and a groove 114 for engagement by a retaining device when the gripper is fitted on a piece of sheet metal. Although the gripper just described is advantageous because of its structural simplicity, grippers of other types, even with mechanical closure, could be used. An alternative type of gripper could be constituted by one fixed arm and one movable arm which could be tightened against the fixed arm by means of a screw operated by an external device independent of the gripper. As explained above, the gripper 98 has to be fitted on to the piece of sheet metal to be bent in a predetermined position. This is achieved by virtue of a positioning table which is described in detail in EP-A- 0 725 692. After the pieces of sheet metal have been provided with their own grippers, and after they have been subjected to a measurement step, they are moved by a manipulator which positions them precisely in a working region. The suspension head 76 of the manipulator, that is, the portion of the manipulator which collects the gripper 98 (which is fixed rigidly to the piece), with the movable device 66 of the manipulator, is a critical component of the operating system which has to perform two conflicting tasks. In fact, whereas, on the one hand, the suspension has to ensure a rigid connection between the gripper for gripping the piece and the movable device of the manipulator so that the piece can be positioned precisely in the working space, on the other hand, during the execution of the bend, the suspension head must leave the piece free to perform slight movements and rotations induced during bending so as not to strain the gripper 98 and the movable device 66 of the manipulator. Figure 5 shows schematically the kinematic behaviour of the suspension head 76. The movable device 66 of the manipulator carries a suspension body 168 by means of resilient weight-compensation means indicated 170. A first pivoting element 172 is articulated to the suspension body 168 and carries, at its lower end, a slide 174 which is movable along the axis X. A second pivoting element 176 is articulated to the slide 174 and carries a rotary body 178 having means for engaging the gripper 98 which is fixed to a piece of sheet metal 180 to be bent. The arrangement described allows the piece of sheet metal 180 five degrees of freedom constituted by three translatory movements along the axes X, Y and Z, a rotary movement about the axis Z and a pivoting movement about the axis X. The piece of sheet metal 180 remains restrained, however, with respect to the last degree of freedom constituted by rotation about the axis Y, which has to be prevented to avoid errors in the positioning of the bending line in the plane of the piece (the plane X-Z). It should be noted that the translation along the axis Y is composed of two pivoting movements of the pivoting elements 172 and 176. The suspension head 76 can be restrained with respect to all the degrees of freedom by means of the clamping and biasing devices for returning the various elements 168-178 constituting the suspension head to a predetermined attitude. Figures 6, 7 and 8 show schematically a practical embodiment of the suspension head, the kinematic layout of which corresponds to Figure 5. The suspension body 168 is guided vertically relative to the movable device 66 by means of a four-bar linkage mechanism comprising a pair of upper connecting rods 182 and a lower connecting rod 184 which are articulated to the suspension body 168 and to the movable device 66 on parallel axes. With reference, in particular, to Figure 7, the resilient balancing means comprise a first spring 186 interposed under compression between the suspension body 168 and the movable device 66, and having a fixed preloading determined so as to balance the weight of the suspension head 76. A second spring 188 is disposed under compression in parallel with the first spring 186 and has a preloading which is variable according to the weight of the piece of sheet-metal 180 connected to the suspension head 76. The second spring 188 is interposed between an upper plate 190 fixed to the suspension body 168 and a head 192 carried by a rod 194 movable along the axis Z and operated by a motor 196 controlled by the control unit of the system, to which data relating to the weight of the pieces to be worked have been supplied beforehand. The first pivoting element 172 is articulated at its upper end on a pin 198 which also acts as an articulation pin for the lower connecting rod 184 of the four-bar linkage mechanism which guides the vertical movement of the suspension body 168. At its lower end, the pivoting element 172 carries a linear guide 200 formed by a roller sliding block extending parallel to the axis of the pin 198 (that is, along the direction (X)). The guide 200 is engaged by the slide 174, which carries a pivot pin 204 which is parallel to the axis of the pin 198 and on which the second pivoting element 176 is articulated. The second pivoting element 176 supports the rotary body 178 by means of bearings 206 with vertical axes. With reference now to Figure 9, a first clamping device, indicated 208, can simultaneously achieve restraint with respect to two degrees of freedom, that is, rotation about the axis X and rotation about the axis Z. The clamping device 208 is housed in the body of the slide 174 and cooperates with the second pivoting element 176 and with the rotary body 178. A second clamping device 210 is interposed between the first pivoting element 172 and the slide 174 for achieving restraint with respect to the degree of freedom corresponding to movement along the axis X. A third clamping device 212 is interposed between the first pivoting element 172 and the suspension body 168 for restraining the element 172 from pivoting about the pin 198. Finally, a fourth clamping device (not visible in Figure 9) is interposed between the suspension body 168 and the movable device 66. The various clamping devices have essentially identical structures and are based on the principle of the gripping between two movable discs a part fixed to a relatively fixed element and a part fixed to a relatively movable element. The structure and operation of the second clamping device 210 will be described with reference to Figures 11 and 12. The clamping device comprises first and second pistons 214 and 216 which are connected to each other by means of a shaft 218 and are mounted for sliding in an airtight manner in a chamber 220 in the first pivoting element 172. A third piston 222 is slidable in a fluid-tight manner both relative to the chamber 220 and relative to the shaft 218. The second and third pistons 216 and 222 act by means of spherical surfaces 224 on respective thrust discs 226 and 228 which are kept separated by a pair of springs 230 and 232. A first region 234 for supply with pressurized fluid (in the specific embodiment, compressed air) is defined between the first and third pistons 214, 222, and a second region 236 for supply with pressurized fluid is defined between the second piston 216 and an end face of the chamber 220. Between the two thrust discs 226, 228 is a portion 238 which forms part of the first pivoting element 172 and in which two calibrated locating pins 240, disposed at 120°, are slidable to keep the opposed faces of the thrust discs 226, 228 a predetermined distance apart. The slide 174 carries rigidly an appendix 242 having a calibrated head 244 which is interposed between the thrust discs 226, 228. When no pressurized fluid is sent to the supply regions 234, 236, the thrust discs 226, 228 are kept in the retracted positions by virtue of the springs 230, 232 and do not exert any restraint on the head 244, and the slide 174 is therefore free to move on the guide 200 along the axis X. In order to clamp the slide 174 with respect to its freedom to move along the axis X, pressurized fluid is supplied to the two supply regions 234, 236 at the same pressure. If the force produced by the pressure acting on each piston is indicated F (the force F is the product of the pressure of the fluid and the surface area of the piston exposed to the pressure), a force equal to 2F will act on the first thrust disc 226, urging it in the direction of the arrow 246, since the forces acting on the first piston 214 and on the second piston 216 are discharged on the first thrust disc 226 by means of the surface 224. A force of magnitude F will act on the second thrust disc 228, urging it in the direction indicated by the arrow 248, due to the action of the piston 222 alone. The first thrust disc 226 thus adopts a stable locating position against the locating surface 238, whilst the second thrust disc 228 presses the calibrated pins 240 and the calibrated head 244 against the first thrust disc 226. The slide 174 is thus brought to a predetermined position which is defined with great repeatability relative to the first pivoting element 172 and remains clamped in that position as long as fluid continues to be supplied to the regions 234, 236. The structure and operation of the rest of the clamping devices are identical to those described with reference to the second clamping device 210 with the sole difference that, in the first clamping device 208, two calibrated heads and a single calibrated pin 240 are disposed between the thrust discs 226 and 228, the heads being disposed in angularly offset positions, one head being fixed to the pivoting element 176 and the other to the rotary body 178, so that the operation of the first clamping device simultaneously clamps the second pivoting element 176 and the rotary body 178 and positions them in a predetermined attitude. The device for connecting a gripper 98 to the suspension head 76 will now be described with reference to Figures 10 and 11. This device is generally indicated 250 and comprises a piston 252 which is slidable in a fluid tight manner in a chamber 254 in an element 256 which forms part of the rotary body 178. The rotary element 178 has a conical seat 258 for housing the shank 106 of the gripper 98. This piston 252 carries six gripping fingers 260 of which only two are visible, which can be moved apart resiliently, and are coaxial with the conical seat 258. Each finger 260 is formed at its lower end with a protrusion 260a having a tapered surface 260b. On the inside of the engagement fingers 260, there is a shaft 262 which is fixed to the element 256. The engagement fingers 260 are intended to engage the seat 110 of the gripper 98 in their spread-out configuration (shown in Figure 11). A helical compression spring 264 is interposed between the rotary body 178 and the piston 252. Two chambers, formed on opposite sides of the piston 252 for supply with pressurised fluid are indicated 266 and 268 respectively. In the configuration of Figure 10, the piston 252 is kept in its lowered position against the thrust of the spring 264 by the pressure of the fluid supplied to the upper chamber 266. In this configuration, the engagement fingers 260 are undeformed and are free of engagement with the seat 110 of the gripper 98 which can therefore be released from the suspension head by a relative movement along the axis Z. The clamping configuration of the connecting device 250 is shown in Figure 11. In this configuration, the pressurised fluid is sent into the chamber 268 and the force of the fluid is added to the thrust of the spring 264. The engagement fingers 260 are moved apart due to their reaction against the shaft 262 and the protrusions 260a engage the seat 110, connecting the gripper 98 firmly to the rotary body 178. It should be noted that the engagement fingers 260 also exert an upward force on the gripper 98, ensuring firm engagement between the conical surfaces of the shank 106 and of the seat 258. Figure 11 also shows the engagement between the angular locating pin 112 of the gripper 98 and a seat 270 of the rotary body 178. An annular spring 271 is interposed between the seat 270 and the pin 112 to take up play.

Claims: A manipulator for gripping and moving an initially flat piece of sheet metal to be subjected to bending operations by means of a bending machine, comprising a suspension head (76) which can allow the piece (180) to perform small movements in space with every degree of freedom except for rotation about an axis perpendicular to the plane of the undeformed piece, characterised in that the suspension head (76) has rapid engagement and release means for gripping and releasing a gripper (98) adapted to grip a piece of sheet metal (180), the suspension head (76) comprising at least one pair of engagement fingers (260) which are movable axially in a conical seat (258) housing a shank (106) of the gripper (98), the engagement fingers (260) being carried by a movable piston (252) operated by pressurised fluid, and cooperating with a fixed shaft (262) which is adapted to move the engagement fingers (260) apart resiliently, the engagement fingers (260) also being able to impart to the gripper (98) a force which tends to insert the shank (106) in the conical seat (258), each finger (260) being formed at its lower end with a protrusion (260a) having a tapered surface (260b) to be engaged with a seat (110) of the gripper (98). A manipulator according to Claim 1, characterised in that the suspension head (76) comprises: a base (66) fixed to a movable structure of the manipulator (64), a suspension body (168) which can perform movements of limited extent in a substantially vertical direction relative to the base (66), resilient balancing means (170) interposed between the base (66) and the suspension body (168), a first pivoting element (172) articulated to the suspension body (168) about a first horizontal axis (198), a slide (174) slidable relative to the first pivoting element (172) along a direction (X) parallel to the first horizontal axis (198), a second pivoting element (176) articulated to the slide (174) about a second horizontal axis (204) parallel to the first, a rotary body (178) connected to the second pivoting element (176) in a manner such that it can rotate freely about an axis (Z) perpendicular to the second horizontal axis (204). A manipulator according to Claim 2, characterised in that the rapid engagement and release means (250) are housed in the rotary body (178). A manipulator according to Claim 2, characterised in that the suspension body (168) is connected to the base (66) by means of a four-bar linkage mechanism. A manipulator according to Claim 2, characterised in that the resilient balancing means comprise a spring (188) with preloading which is adjustable in dependence on the weight of the piece of sheet-metal (180) to be picked up. A manipulator according to Claim 2, characterised in that it comprises a plurality of clamping and biasing devices (208, 210, 212) for restricting the degrees of freedom of the suspension head (76) and returning the elements constituting the suspension head (76) to a predetermined attitude. A manipulator according to Claim 6, characterised in that each clamping and biasing device is interposed between a relatively fixed part (172) and a relatively movable part (174) and comprises a chamber (220) in one of the parts (172, 174), first and second discs (226, 228) being slidable and movable towards each other in the chamber (220) under the action of pressurised fluid, and being able to clamp between them an appendage (242) carried by the other of the relatively fixed and movable parts. A manipulator according to Claim 7, characterised in that a first disc (226) is urged against a shoulder (238) fixed to the part in which the chamber (220) is formed, by a force which is substantially double that which urges the second disc (228) against the shoulder. A manipulator according to Claim 8, characterised in that the first and second discs (226, 228) are operated by an actuator device comprising first and second pistons (214, 216) which are fixed together and can slide freely in the chamber (220), and a third piston (222) which can slide in the chamber independently of the first two, a first pressurised-fluid supply region (234) being defined between the first and third pistons (214, 222) and a second pressurised-fluid supply region (236) being defied between the second piston (216) and an end of the chamber (220). A manipulator according to Claim 1, comprising a plurality of clamping and biasing devices (208, 210, 212) for restricting the degrees of freedom of the suspension head (76) and returning the elements constituting the suspension head (76) to a predetermined attitude. A manipulator according to any of the preceding Claims, characterised by a feeler (88) for measuring the piece which is connected to the manipulator (64).

Inventors: SARTORIO FRANCO, SARTORIO, FRANCO
Applicants: AMADA CO LTD, CREA SRL, AMADA COMPANY, LIMITED, CREA S.R.L.
Application year: 1994
Main classification: B21D11
IPC codes: B21D37, B21D5, B21D43, B21D11